Field of Invention
This invention relates to a novel apparatus for testing shape memory effects in liquid bath. The present invention provides a means of measuring shape memory effects under harsh environments including hot, cold, corrosive, non-corrosive, chemically reactive, acidic, basic, neutral liquid mediums. It is capable of measuring mechanical properties including recovery force of the shape memory polymers and composites in an efficient but inexpensive and simple arrangement under controlled temperature and in different liquids/environment conditions.
Related Art
Shape Memory Materials are a broad class of smart materials which are capable of holding a temporary shape until a suitable stimulus is applied after which they regain their original permanent shape. Several materials exhibit the Shape Memory Effect (SME), such as Shape Memory Alloys (SMA), Shape Memory Polymers (SMP), Shape Memory Ceramics and Shape Memory Gels. SME is reported in terms of strain recovery rate, stress recovery rate, shape fixity, etc as reference could be made to a publication “Challenges of shape memory polymers: A review of the progress toward overcoming SMP's limitations, published in Polymer Engineering & Science, Volume 48, Issue 11, pages 2075-2089, November 2008 by John Wiley & Sons, Inc. This document contains state of art reviews and experimental studies related to shape memory polymers. A thermo-mechanical programmed cycle describes well the effect of temperature on stress and strain developed within the specimen. Normally thermo-mechanical cycles for Shape Memory Polymer are studied in an environmental chamber having air or nitrogen circulation. Heat is provided generally by electrical heaters and cooling is done either through forced circulation of air or by using liquid nitrogen circulation arrangement. The thermo-mechanical programmed cycle used for evaluation of shape memory effects of materials consists of following steps; in the first step the sample is fixed at a standard gauge length which is named as original length (lo) and the environmental chamber is brought to a high temperature Th which is chosen above the glass transition temperature, Tg, of polymer. The sample is stretched at predetermined strain and the corresponding force is measured by the load cell and thereby calculating the stress on the sample. The second step involves the cooling of the sample while maintaining the strain, to room temperature by either of natural cooling or by forced cooling using liquid nitrogen or any other suitable method by simply cooling the supplied air. Once the sample is cooled, it is unclamped. The length of relaxed sample is measured and termed as deformed length (ld). In third step the sample is clamped and temperature of environmental chamber is brought to Th. The recovery stress generated in the sample due to its shape memory effect is measured. The strain on the sample is decreased and corresponding stresses are measured. Finally the recovered length is measured at almost zero value of recovery stress.
Presently tensile testing of materials is one of the most common ways to measure material dependent properties. Such tests can give almost any material data a designer needs to know.
For tensile testing the specimen is normally mounted between two attachments. One of the attachments is normally arranged at a frame, while the other is arranged at a movable pulling yoke. The pulling yoke is moved so as to elongate the tensile testing specimen, which finally breaks. The relation between strain and tensile force may be recorded as the pulling continues. The standard apparatus are used all over the world for measuring mechanical properties of different materials such as metals, ceramics, polymers, composites, and others. However, for the measurement of specific properties of materials, different arrangements, fixtures, test jigs etc. are used for customize testing.
U.S. Pat. No. 6,148,676 of Tensile testing machine can be referred as an apparatus developed for metallic materials, which measures the strain for a tensile testing specimen by means of the relative displacement, which is independent of the applied tensile testing force. U.S. Pat. No. 8,065,929 can be referred as visual mechanical test apparatus that includes a pressure generation source and an isolation cylinder which is used to measure the mechanical properties of materials under high pressure conditions. The test apparatus further includes a pressure containment cell configured to receive a test specimen, an environmental chamber disposed about the pressure containment cell, and a load frame operably associated with the pressure containment cell. The pressure containment cell is in fluid communication with the pressure generation source and the isolation cylinder and includes a viewing window. U.S. Pat. No. 7,191,664 can be referred as apparatus for testing of mechanical properties of materials for use in pavement construction. The sample container has a first open face via which a compressive load can be applied to the sample and a second face disposed orthogonally to the first open face. At least a portion of the second face is resiliently displaceable. U.S. Pat. No. 7,240,545 can be referred for measurement of expansion and contraction of sample curing under pressure and temperature.
Normally shape memory effects such as recovery stress, shape fixity, percentage recovery, etc. are measured above glass transition temperature of the material for which environment chamber having inert environment or specific environment is used. For the second programmed steps, the temperature of the chamber including grips and specimen is to be brought to below glass transition temperature to enable the sample to reach a temporary shape. The attainment of the lower temperature by the specimen as well as by the grips is a time consuming phenomena. For the third programmed steps of the thermo mechanical shape memory creation process, the temperature of the environment chamber is increased to a temperature higher than the glass transition temperature as attained in the first step. After measuring the stress and corresponding strain in the sample the temperature is again brought to below the glass transition temperature. In this process the temperature conditions are very important to characterize the shape memory effects. The problem associated with the present practice in the measurement of shape memory effects is its inevitable time consuming process. Most of the time goes in cooling and heating the environmental chamber and its accessories including sample grips and the test specimen. Delayed cooling and prolonged heating in environment chamber of a tensile testing machine may cause deterioration and degradation of the material to be tested. Moreover, the tensile testing machine having environment chamber are not capable of testing the material under liquid environment of harsh fluid environment